An image converter for amplifying the light intensity of an intensity distribution of an electromagnetic radiation is known from DE-Patent 535 208. A photoelectric layer is provided as a converter layer, out of which the electromagnetic radiation releases electrons, and which at the same time serves as a cathode of an adjacent flat tube. In the tube, the released electrons are accelerated against an anode, which emits light due to the incident electrons, the light being usable for illuminating an adjacent photographic layer. The spectral properties of the photoelectric layer define the spectral sensitivity range of the image converter, which is limited to 200 to 1300 nm and which often is much narrower.
The image converter known from DE-Patent 535 208 is, for example, used in so-called night vision devices. Infrared light not visible to the human eye or residual light insufficient for the human eye is made visible to the human eye with night vision devices, the accelerated and usually additionally amplified electrons being caught by a luminescence screen consisting of material luminescent in the visible range. Here, the electrons activate the luminescence, the distribution of which over the luminescence screen corresponds to the intensity distribution of the infrared or residual light incident on the photoelectric layer. The luminescence screen can be directly viewed with the eye or imaged onto an image sensor array.
In a variant of the image converter known from DE-Patent 535 208 which has been developed by the company PROXITRONIC in Darmstadt, light which is to be made visible is imaged onto a flat tube in which a suitable gas is subject to high voltage. The light incident in the tube results in an ionization of the gas. Due to the applied high voltage, the electrons of the ionized gas are accelerated towards a luminescence screen and cause luminescence, the distribution of which over the luminescence screen corresponds to the intensity distribution of the light to be made visible incident on the flat tube.
All image converters based on the general principle known from DE-Patent 535 208 are only designed for very low intensities of the viewed electromagnetic radiation and thus not suitable for visualization of an intensity distribution of a stronger electromagnetic radiation. To nevertheless enable them for this purpose, the incident electromagnetic radiation has to be strongly artificially attenuated. Doing this, however, the sensitivity of the image converter for weak intensity gets lost.
A further image converter working according to the general principle known from DE-Patent 535 208 is known from U.S. Pat. No. 2,572,494. This image converter comprises a heating device for raising the temperature of the photoelectric layer out of which the electric radiation releases electrons into the range of 40° C. or 50° C. At this temperature the electron current through the acceleration tube shall be in the order of 10−10 A/cm2. I.e. the increase in temperature provides a minimum electron current through the tube.
Further image converters are known in which the electromagnetic radiation the intensity distribution of which is to be made visible is directly incident on a luminescence screen used as a converter layer. Here, the electromagnetic layer may not only activate luminescence, but alternatively also deactivate an existing luminescence by stimulated emission resulting in a negative image of the intensity distribution of interest. The activation of the luminescence may be due to a quantum optics excitation of the luminescence screen or due to a quantum optics or thermal stimulation of a luminescence screen preloaded with short wave radiation. All luminescence screens with luminescence excited or de-excited by quantum-optical effects only have a small banded spectral sensitivity range.
Image converters having a so-called microbolometer array make use of temperature dependent electrical properties of individual elements of the microbolometer array to register a temperature distribution over the elements of the microbolometer array caused by an electromagnetic radiation incident on the microbolometer array.
It is known to apply thermochrome paint layers, for example made of liquid crystals, onto surfaces to visualize temperature distributions over surfaces.
In an image converter known from DE 101 58 859 B4, the converter layer is the reflecting coating of a laser mirror. The temperature distribution over the reflection layer caused by the incident laser radiation which is essentially reflected by the reflection layer is made visible in that the reflection layer is arranged on an expansion layer which expands depending on its temperature, and in that the resulting deformations of the reflection layer due to local thermal expansions of the expansion layer are registered with a laser interferometer. No direct image of the intensity distribution of the laser radiation is obtained in this way, and high efforts have to be taken to at all register the local thermal expansions of the expansion layer occurring at a very low level. Vice versa, the known image converter having the features of the preamble of claim 1 is suitable for a high bandwidth of intensities of the incident electromagnetic radiation inclusive of laser radiation of high local intensity, and it has a big spectral sensitivity range.